The present invention relates to the measurement of optical characteristics of an optical device under test, and more particularly to a method and apparatus for measuring multiple optical characteristics in a single sweep of a swept wavelength system using Jones Matrix Eigen Analysis.
It is well known in the art that the Jones matrix of an arbitrary two-port optical device may be measured by using three known input states of polarization and measuring the resulting output states of polarization. Polarized light is represented by a two-element complex vector, i.e., the Jones vector, the elements of which specify the magnitude and phase of the x- and y-components of the electric field at a particular point in space. The Jones matrix for the optical device relates the input and output Jones vectors to each other. The Jones matrix representation is found by measuring three output Jones vectors in response to three known input stimulus states of polarization, or input Jones vectors. Fiber Optic Test and Measurement, Dennis Derickson, Prentice Hall, 1998, page 225. The mathematical calculations are simplest when the stimuli are linear polarizations oriented at zero, forty-five and ninety degrees as shown in FIG. 1, but any three distinct stimuli may be used.
Using the convention shown in FIG. 1 the Jones matrix of an optical device under test (DUT) at a particular optical frequency is calculated from the following equation:   J  =      C    ⁡          [                                                  K1              *              K4                                            K2                                                K4                                1                              ]      where the different components of the Jones matrix are given by:K1=[X1/Y1] K2=[X2/Y2] K3=[X3/Y3] K4=[(K3−K2)/(K1−K3)]and J[X1,Y1] is the output Jones vector for the input linear-horizontal state of polarization, J[X2, Y2] is the output Jones vector for the input linear-vertical state of polarization, and J[X3, Y3] is the output Jones vector for the input linear-forty-five degree state of polarization. In the Jones matrix equation the factor C is a constant phase/amplitude multiplier that is undetermined and unnecessary for measuring polarization-dependent loss (PDL) or polarization differential group delay (DGD). In practice the output Stokes vector is measured and then the Jones vector is calculated, as is well-known to those skilled in the optical arts as shown in the Derickson text book cited above.
Also it is well known that the wavelength-dependent Jones matrix may be measured by sweeping over a wavelength range using a fixed input horizontal state of polarization while measuring the output state of polarization at each wavelength increment; then sweeping over the same wavelength range using a different fixed input vertical state of polarization while measuring the output state of polarization at each wavelength; and sweeping a third time over the same wavelength range using yet another fixed input state of polarization while measuring the output state of polarization. Then by correctly registering the sweeps from the various output states of polarization with the same wavelengths for each sweep, the Jones matrix is calculated at each wavelength using the equations above.
Further it is well known that one may measure the three output states of polarization for three different input states of polarization at a fixed wavelength, and then calculate the Jones matrix at that wavelength. The wavelength may then be indexed and the process repeated to calculate the Jones matrix as a function of wavelength. Knowing the Jones matrix as a function of wavelength is important because it allows the determination of wavelength dependent optical characteristics such as polarization-dependent loss (PDL) and polarization dependent group delay (DGD). These are important characteristics of optical devices, and help to determine the degree to which the optical device may degrade an optical telecommunications system. Given the Jones matrix the PDL may be found from:PDL=10*Log(λ1/λ2) where λ1 and λ2 are the eigenvalues of (J*)TJ. The DGD is also found from the Jones matrix as:DGD(ω)=∥arg(ρ1/ρ2)/Δω|where ρ1 and ρ2 are the eigenvalues of J(ω+Δω)*J−1(ω).
It is obvious from these descriptions that the testing over wavelength is slow. The first process requires three different scans over a wavelength range. If there are N wavelengths in each scan, then the first method requires the measurement of N*3 output states of polarization. The second method steps through the wavelengths only once, but this must be a stepping motion with a pause at each wavelength to measure the three different states of polarization. Again the number of output states of polarization is N*3.
What is desired is a faster method of measuring multiple optical characteristics of an optical device that requires fewer measurements of output states of polarization, and more specifically a method of scanning over a wavelength range once to determine the wavelength-dependent Jones matrix of the optical device from which the multiple optical characteristics are calculated simultaneously.